In the field of haptic interfaces, notably force feedback devices, for example an orthosis intended to rehabilitate the motor capabilities of a patient, it is common practice to use a motor to exert a force or a torque that simulates the resistance of the environment to the movement of the device. To simulate a virtual contact, for example, an electric motor is controlled so that it exerts on an output member of the device a resisting torque that opposes the movement of the device required by the operator.
To simulate contact with a virtual rigid surface, the motor must react as a real rigid surface would. Unfortunately, it is known that a powerful motor generally has high friction torque, which limits its capacity to produce forces of low amplitude for simulating an unrestricted movement or the feel of a texture. Moreover, such a motor has high inertia that may be felt by the operator.
To compensate this friction and this inertia, a force servo-control loop may be used employing a torque meter to measure directly the torque at the output of the actuator. In addition to the construction difficulties inherent to torque meters, their location in the dynamic system is critical to obtaining a good compromise between stability and precision.
A less powerful motor may be used having low friction and low inertia. However, such a motor loaded in this way would often be operating at the limit of its capabilities and would therefore have a limited service life.